mDNS provide hostname resolving for domain .local. Every device which support mDNS have domain <hostname>.local. So when computer (with mDNS support) and Nokia N900 connect to wifi network, mDNS can be used for identifing network devices and resolving hostname to ip addresses. Usefull for SSH when DHCP server assigning random ip address.

Sometimes (or always), after boot, avahi-daemon not starting.
When I "sudo start avahi-daemon", it says about too frequent starting.
But if I "sudo avahi-daemon", it starts normally. Then Ctrl-C, and "sudo start avahi-daemon" now ok.

sudo start avahi-daemon - if some time passed after reboot - normal start. I think some service prevents start of avahi-daemon on startup (at least on my phone, many programs installed). Maybe it will be enough to start after hildon-desktop e.g., not after dbus.